Referring to FIG. 1, a brushed electrical machine 1 comprises a rotor 6, a stator 7, a commutator 8 and brushes 9. The brushes 9 are attached to brush bridges 10 and their purpose is to close a current circuit to the rotor windings through the commutator 8. During the operation of the machine 1, heat is generated in the windings of the machine due to copper losses, in the rotor and stator cores due to iron losses, and in the commutator 8 due to friction losses and electrical losses at the interface of the brushes 9 and the commutator 8. The machine 1 therefore has to be cooled to keep the temperature of the machine parts at an acceptable level and to thereby ensure a reliable operation of the machine 1. Cooling of an electrical machine 1 according to FIG. 1 is normally accomplished with a cooling system comprising a cooler, in this case a fan 2 that blows air through the machine 1. In a typical arrangement the fan 2 has an auxiliary motor 3 of induction type which rotates the fan 2 with a constant speed. The cooling air is drawn into the machine 1 through an air inlet 4, and after flowing through the interior of the machine 1 the air is exhausted into the ambient of the machine 1 through an air outlet 5.
In order to ensure a proper cooling under all operating conditions, the cooling effect of the cooler has to be dimensioned to correspond to the maximum load of the machine 1 run at a maximum allowed ambient temperature. In the case of the cooler according to FIG. 1, running the fan 2 at a constant speed leads to a considerably low machine temperature under many operating conditions, particularly so under low load conditions and under operation in cold environment.
In order to improve the energy efficiency of a cooling system comprising a fan and an auxiliary motor, it is a known technology to provide the auxiliary motor with a variable speed drive and to adjust the rotational speed of the fan in relation to a measured or estimated machine temperature. A control system wherein a variable speed drive generates a control signal in accordance to a temperature signal detected by a temperature sensor that is connected to the machine is known e.g. from JP62037037.
A method for controlling the temperature within an electrical device by adjusting the rotational speed of a fan in relation to an electrical load of the electric device, ambient temperature and an air flow past the electrical device is known from U.S. Pat. No. 5,777,897.
As is well known to a person skilled in the art, most typical commutator brushes are made of carbon, usually together with some form of additive compound for increased performance and operation under special conditions. One problem related to brushed machines is brush wear which is strongly dependant on the commutator temperature. Both too low and too high temperatures lead to an extensive brush wear, the increase of the wear being dramatic at low temperatures.
Because the known temperature controlling methods tend to emphasize the reduction of copper losses that decrease with a decreasing temperature, the reference temperature is many times chosen to be relatively low. From the brush wear point of view the conventional temperature controlling methods often result to too high cooling effect such that the low commutator temperature causes an unnecessary high brush wear rate. Too high cooling effect also leads to unnecessary high energy consumption in the auxiliary motor, and the energy efficiency of the overall combination of the brushed electrical machine and the auxiliary motor is worsened. In other situations the cooling effect can be adjusted to be too low, causing the commutator to overheat. The overheating may occur especially shortly after the start-up of the machine when the commutator warms up faster than the magnetically active parts.
A device for lowering especially the surface temperature of a commutator and brushes is known from JP2001169511 which discloses a fan that is configured to automatically rotate together with a rotor shaft when the temperature around the commutator is higher than predetermined.